The present invention relates to a process for making using a substituted succinic anhydride as a sizing agent, which can provide a preferable sizing effect to be achieved by a substituted succinic anhydride and high retentions of a fine fiber and a filler in a pulp slurry.
There is known a process for making paper employing as a sizing agent a substituted succinic anhydride, and a process for sizing paper by adding an aqueous dispersion, prepared by dispersing a substituted succinic anhydride in water with the aid of a suitable dispersant, to a pulp slurry.
There are also known as a dispersant, in the case of preparing the aqueous dispersion of the substituted succinic anhydride, processes employing a cationic starch (disclosed in Japanese Patent Publication No. 2305/1964, Japanese Patent Laid-open No. 197397/1978 official gazettes), a cationic or ampholytic synthetic polymer substance (disclosed in Japanese Patent Laid-open No. 45730/1983, Japanese Patent Laid-open No. 120897/1983 official gazettes), and various surface active substances (disclosed in Japanese Patent Publication No. 36044/1978, Japanese Patent Laid-open No. 87397/1983, Japanese Patent Laid-open No. 220897/1983, Japanese Patent Laid-open No. 47498/1984, Japanese Patent Laid-open No. 187696/1984, Japanese Patent Laid-open No. 28598/1985 official gazettes).
It is, on the other hand, known to add a retention aid in a pulp slurry to improve the retention of fine fiber and a filler in the pulp slurry in a process for forming a sheet of paper from the pulp slurry. For example, there is know as a general retention aid cationic polymer substances such as cationic starch, cationic polyacrylamide, polyethylene-imine, polyamide-polyamine, etc. Recently, a process for making paper for remarkably improving the retention of a fine fiber and a filler in the pulp slurry by employing both a cationic starch or cationic or ampholytic Guar Gum and a colloidal silica has been proposed (disclosed in Japanese Patent Laid-open No. 51900/1982, Japanese Patent National Publication No. 502004/1983 official gazettes).
The inventors of the present invention have studied the operation and the advantages of improving the retentions of a fine fiber and a filler in a pulp slurry prepared by adding a colloidal silica in the pulp slurry in a sheet formation. It has been confirmed that, when the colloidal silica is added in the pulp slurry after sizing paper by adding an aqueous dispersion of a substituted succinic anhydride in the pulp slurry, the retentions of a fine fiber and a filler in the pulp slurry in a sheet formation are significantly improved, but the inventors have also discovered that the sizing effect of the prepared paper exhibited much more deterioration than when the colloidal silica is not added. In a process for making paper using as a papermaking sizing agent a substituted succinic anhydride, the addition of the colloidal silica as a retention aid of the fine fiber and the filler of the pulp slurry causes the sizing effect of the substituted succinic anhydride to decrease. The operation for decreasing the sizing performance of the substituted succinic anhydride with the colloidal silica becomes higher when the steps of adding the aqueous dispersion of the substituted succinic anhydride in the pulp slurry and the step of then adding the colloidal silica become closer. If these steps are reversed in sequence for adding them in the pulp slurry, the decrease in the sizing performance is suppressed to a relatively low value, but cannot be completely eliminated. Other problems such as the decrease in the retention aiding action of a fine fiber and a filler in the pulp slurry with the colloidal silica and the decrease in the drainage of the pulp slurry have been discovered when the decreasing action of the sizing performance of the substituted succinic anhydride with the colloidal silica is suppressed to a low value.